FILM NEGATIVE PROCESS, or film photography, is a term applied to processes in which flexible films are used instead of glass plates. As far back as 1855 Scott Archer patented a method of coating collodion negatives with a solution of gutta-percha in benzole. When dry the film was separated from the glass by soaking in water. The glass might also be made to receive a preliminary coating of gutta-percha, the effect of which would be to enclose the collodion film between two waterproof films. The late W. B. Woodbury used during the collodion days a method by which glass was superseded in part. A sheet of glass was first rubbed over with powdered talc, and then coated with a collodion emulsion. After exposure these plates were developed, fixed, and washed, and a sheet of gelatine paper was squeegeed to the finished nega tive. When dry, the whole could be peeled off the glass, which was again used.
The recent advances in film photography consist in the coating of paper, insoluble gelatine, celluloid, or other flexible material with the sensitive bromide gelatine emulsion. These films are then exposed in the camera, developed, and fixed in the same manner as dry plates. Their principal advantage over glass is, of course, in their lightness, and they have been largely used by tourists and travellers. They have also the advantage of being flexible, unbreakable, and can be packed in a very small space. In the photography of brightly lit objects they are superior to
glass, as halation is avoided. Another advantage is that prints can be made from either side with equal facility—a great advantage to the carbon or collotype printer.
The best kind of films are those made upon insoluble gelatine or celluloid. To make the former, clean a large sheet of glass and rub it well with French chalk. A thick solution of gelatine is then poured over it, and the plate placed on a levelling stand to set. When dry the plate is placed in a saturated solultion of chrome alum. This effectually renders the gelatine insoluble. It is then washed, dried, and removed to the dark room to receive the dry-plate emulsion in the manner described under dry-plate. When dry the film is stripped from the glass, and cut up to the required sizes. Instead of chrome alum, the gelatine film may be hardened by soaking with potassium dicromate, exposing to the light, and then well washing; but I believe this method is covered with a patent.
Sensitive films are now made in lengths, and can be used in suitable holders, so that a roll of, say, 25 exposures can be obtained without changing. (See also under Paper Negative, Strip ping Films.)